U.S. patent application number 10/989436 was filed with the patent office on 2006-05-18 for digital potentiometer.
This patent application is currently assigned to Avistar, Inc.. Invention is credited to Michael Garcia, Gary Kessler, Albert Migliori.
Application Number | 20060103500 10/989436 |
Document ID | / |
Family ID | 36385683 |
Filed Date | 2006-05-18 |
United States Patent
Application |
20060103500 |
Kind Code |
A1 |
Kessler; Gary ; et
al. |
May 18, 2006 |
Digital potentiometer
Abstract
A variable potentiometer with a wiper terminal and first and
second terminals has relay switches for shorting or for unshorting
resistors. When resistance is reduced between the wiper and one of
said terminals, resistance is increased between the wiper and
another terminal. In one embodiment two strings of resistors with
the same nominal values are used between the wiper and the
terminals. In another embodiment, a single string of resistors are
used and are switched into either the electrical connection between
the wiper and the first terminal or between the wiper and the
second terminal. When resistance is lowered between the wiper and
one of said first or second terminals a first resistor is replaced
with a first short circuit and when resistance is increased between
said wiper and another of said first and second terminals a second
short circuit is replaced with the first resistor.
Inventors: |
Kessler; Gary; (Albuquerque,
NM) ; Garcia; Michael; (Albuquerque, NM) ;
Migliori; Albert; (Sante Fe, NM) |
Correspondence
Address: |
SNIDER & ASSOCIATES
P. O. BOX 27613
WASHINGTON
DC
20038-7613
US
|
Assignee: |
Avistar, Inc.
Albuquerque
NM
|
Family ID: |
36385683 |
Appl. No.: |
10/989436 |
Filed: |
November 17, 2004 |
Current U.S.
Class: |
338/195 |
Current CPC
Class: |
H01C 10/08 20130101 |
Class at
Publication: |
338/195 |
International
Class: |
H01C 10/00 20060101
H01C010/00 |
Claims
1. A variable potentiometer comprising in combination: a wiper
terminal; a first terminal electrically connected to said wiper
terminal; a second terminal electrically connected to said wiper
terminal; a plurality of switches for changing resistance between
said wiper terminal and each of said first and second terminals;
wherein when resistance is reduced between said wiper and one of
said first or second terminals a first resistor is replaced with a
first short circuit; and wherein when resistance is increased
between said wiper and another of said first and second terminals a
second short circuit is replaced with the first resistor.
2. The variable potentiometer in accordance with claim 1, wherein
said switches comprise relays which replace the first resistor with
a short circuit and replace a short circuit with the first
resistor.
3. The variable potentiometer in accordance with claim 1, wherein
when resistance is changed between said wiper and one of said first
and second terminals; a first double pole double throw relay
electrically removes the resistor from the electrical connection
between one of said first and second terminals; a second double
pole double throw relay inserts a shorting connection in the
electrical connection between the one of said first and second
terminals; the first double pole double throw relay electrically
inserts the resistor between the other of said first and second
terminals; and the second double pole double throw relay
electrically removes a short connection between the other of said
first and second terminals.
4. The variable potentiometer in accordance with claim 3, wherein a
four pole double throw relay comprises the first and second double
pole double throw relays.
5. The variable potentiometer in accordance with claim 1, wherein
when resistance is changed between said wiper and one of said first
and second terminals; a first double pole double throw relay
electrically removes the resistor from the electrical connection
between one of said first and second terminals and replaces the
resistor with a short; a second double pole double throw relay
electrically inserts the resistor and removes a short in the
electrical connection between the other one of said first and
second terminals.
6. The variable potentiometer in accordance with claim 5, wherein a
four pole double pole relay comprises the first and second double
pole double pole relays.
7. A variable potentiometer comprising in combination: a set of
resistors; a wiper terminal; a first terminal electrically
connected to said wiper terminal; a second terminal electrically
connected to said wiper terminal; a plurality of switches for
changing resistance between said wiper terminal and each of said
first and second terminals; wherein when resistance is reduced
between said wiper and one of said first or second terminals a
subset of said resistors is replaced with first short circuits; and
wherein when resistance is increased between said wiper and another
of said first and second terminals second short circuits are
replaced with the subset of resistors.
8. The variable potentiometer in accordance with claim 7, wherein
said switches comprise relays which replace the subset of resistors
with first short circuits and replace second short circuits with
the subset of resistors.
9. The variable potentiometer in accordance with claim 8, wherein
said relays comprise double pole double throw relays.
10. The variable potentiometer in accordance with claim 8, wherein
said relays comprise four pole double throw relays.
11. The variable potentiometer in accordance with claim 7, wherein
when resistance is changed between said wiper and one of said first
and second terminals; a first subset of double pole double throw
relays electrically remove the subset of resistors from the
electrical connection between one of said first and second
terminals; a second subset of double pole double throw relays
insert a subset of shorting connections in the electrical
connection between the one of said first and second terminals; the
first subset of double pole double throw double throw relays
electrically inserts the subset of resistors between the other of
said first and second terminals; and the second subset of double
pole double throw relays remove a subset of short connections
between the other of said first and second terminals.
12. The variable potentiometer in accordance with claim 11, wherein
four pole double pole relays comprise the first and second subsets
of double pole double pole relays.
13. The variable potentiometer in accordance with claim 7, wherein
when resistance is changed between said wiper and one of said first
and second terminals; a subset of first double pole double throw
relays which electrically remove a resistor subset from the
electrical connection between one of said first and second
terminals and replace the resistor subset with a subset of shorts;
a subset of second double pole double throw relays which
electrically insert the resistor subset and remove a subset of
shorts in the electrical connection between the other one of said
first and second terminals.
14. The variable potentiometer in accordance with claim 13, wherein
a four pole double throw relay comprises the first and second
double pole double throw relays.
15. The variable potentiometer in accordance with claim 7, wherein
the resistors in the set of resistors have a tolerance value of 1%
or less.
16. The variable potentiometer in accordance with claim 7, wherein
resistors in the set of resistors have a tolerance value of which
is determined so that where a smallest resistor value is R1 and a
largest resistor value is Rn, then: Rn.times.(TOLERANCE)<R1
17. The variable potentiometer in accordance with claim 7, wherein
the sum of the set of resistor values is constant and the total
resistance between said first and second terminals always
constant.
18. The variable potentiometer in accordance with claim 7, further
comprising a computer for controlling the plurality of
switches.
19. The variable potentiometer in accordance with claim 7, further
comprising a computer having a display which provides for a
rotating pointer within a circle which is controlled by a
mouse.
20. The variable potentiometer in accordance with claim 11, wherein
said computer display comprises a display circle that is calibrated
around its perimeter to indicate a quantity controlled.
21. The variable potentiometer in accordance with claim 8, wherein
resistor subsets have a value other than zero and there are
resistor subsets between said wiper and said first terminal and
between said wiper and said second terminal-when said relays are in
their normally closed position.
22. A variable potentiometer comprising in combination: a wiper
terminal; a first terminal electrically connected to said wiper
terminal by a first group of resistors; a second terminal
electrically connected to said wiper terminal by a second group of
resistors; a plurality of switches for changing resistance between
said wiper terminal and each of said first terminal and second
terminal; wherein resistance is changed between said wiper and said
first terminal by placing a short circuit across one or more of
said first group of resistors or by removing a short circuit from
across one or more of said second first of resistors; wherein
resistance is changed between said wiper and said second terminal
by placing a short circuit across one or more of said second group
of resistors or by removing a short circuit from across one or more
of said second group of resistors; and wherein resistors of said
first group and resistors of said second group have values which
correspond to each other.
23. The variable potentiometer in accordance with claim 22, further
comprising a computer for controlling the plurality of
switches.
24. The variable potentiometer in accordance with claim 22, wherein
said switches comprise double pole/double throw relays.
25. The variable potentiometer in accordance with claim 22, wherein
when a resistor in said first group is short circuited, the
corresponding resistor in the second group has a short removed.
26. The variable potentiometer in accordance with claim 22, wherein
when resistance is changed between said wiper and one of said first
and second terminals; a subset of double pole double throw relays
electrically remove a subset of resistors from the electrical
connection between the wiper and one of said first and second
terminals, insert a corresponding subset of shorting connections in
the electrical connection between said wiper and one of said first
and second terminals, electrically insert another subset of
resistors between the wiper and the other of said first and second
terminals, and remove a corresponding subset of shorting
connections between the wiper and the other of said first and
second terminals.
27. The variable potentiometer in accordance with claim 22, wherein
resistors in the first and second groups of resistors are resistors
having a tolerance value which is determined so that where the
smallest resistor is R1 and the largest resistor is Rn, then:
Rn.times.(TOLERANCE)<R1
Description
TECHNICAL FIELD OF THE INVENTION
[0001] This invention relates to potentiometer circuits of the type
where there is a wiper and first and second terminals at each end
of a resistance between the terminals. In early potentiometers, the
resistance may have been wire wound or a resistance pad. In these
devices a wiper moved along the resistor and picked off the voltage
from the resistor. These devices find application in high current
applications. As a practical matter, impedance at the wiper would
be high which would prevent current drain at the wiper. In these
prior art devices, the wiper is physically moved by an
operator.
PRIOR ART
[0002] Prior art devices generally utilize a resistor chain with
taps between each pair of resistors. U.S. Pat. No. 4,849,903 to
Fletcher and Ross is hereby incorporated by reference and teaches a
digitally controlled variable resistor for effecting and presenting
a selected electrical resistance. Resistance may be changed along a
string of resistors by shorting out the particular resistor with a
field effect device. However, such devices, since they are
dependent upon field effect devices for shorting the resistor, are
necessarily not useful in alternating current applications. In the
U.S. Pat. No. 4,849,903, across each resistor (24a-24L) is a series
of field effect device (54a-54L). When a device, such as (54a) is
turned on, a resistor (24a) is effectively short circuited or
removed from the series group of resistors or string of resistors.
The resistors (24a -24L) are chosen in accordance with Table 2 and
increase in value in accordance with an expression 2.sup.N-1(R)
where R is equal to a selected electrical resistance. With this
scheme, shorting out any combination of resistors (24a-24L) allows
for selection of different values of resistance in increments of 1
ohm to 4095 ohms. However, the '903 patent includes a resistance
(24a) of a value of 1 ohm and another resistance of 2048 ohms on
resistor (24L). This means that very close tolerances are required
for all resistors in order for changes in total resistance to be
accurate. If the tolerance of a large resistor has a value greater
than the smallest resister, value and resistance may decrease when
an increase is intended.
[0003] U.S. Pat. No. 4,849,903 in FIG. 1 shows a control scheme for
switching on and off optoisolators (36a) each of which include an
LED (38) and anode (40). When any combination of lines to the
optoisolators change state, the value of the resistance between
terminal (57) and terminal (60) will change to a selected value.
One or more lines from the computer may be selected in order to
select a particular resistance value. In this disclosure, there is
disclosed both a digital computer (17) for selecting the lines
(19a-19L) to resistors (24a-24L) as well as an analog to digital
converter (18). The control system of the U.S. Pat. No. 4,849,903
may be used with Applicant's invention to change the state of
relays which control shorting of resistors. This control can be
used to select a single relay, a double pole double throw or a four
pole double throw relay in accordance with embodiments of
applicant's invention.
[0004] Potentiometer circuits which digitally controlled taps or
connections between pairs of resistors in a string of resistors are
known in the art. U.S. Pat. Nos. 5,297,056 and 5,243,535 owned by
Dallas Semiconductor are examples of this type of digital
potentiometer, and are incorporated here in by reference. These
devices use strings of resistors which are identical in value and
have a "wiper" which is a plurality of switchable taps located
between each pair of resistors. Each of these patents shows a
digital potentiometer which has a plurality of resistors and at the
low end of the potentiometer the "wiper" may be connected to a "LO"
terminal directly.
[0005] In both the '056 and '535 patents, a wiper contact is
provided where a contact point is selected between any two
resistors in a string as shown in FIG. 8. These patents never
connect more than one wiper contact in a passive resistor string.
Therefore, there is no ability to short circuit any resistor in the
string and therefore the resistance from the "LO" to the "HI"
terminals never changes.
[0006] The potentiometer of both '056 and '535 maintains a constant
value between the low terminal and the high terminal. There is no
capability of changing the overall value of the string of
resistors. The wiper is merely moved from string tap point to
string tap point to select different resistances between the wiper
and the low and high terminals. As disclosed, each resistor in a
string of 256 resistors has the same value. This approach is
analogous to a simple linear wire wound potentiometer, except that
it selects taps with FET devices which are selected by a computer
instead of a mechanical contact manipulated by hand.
SUMMARY OF THE INVENTION
[0007] This invention utilizes switches (preferably mechanical
relays) which are controlled in any combination of on-off states by
a computer. The switches are generally in the form of relays
because the invention is not dependent upon field effect
transistors. FET devices as taught by the prior art cannot respond
to alternating currents or reverse currents.
[0008] In accordance with this invention, applicant uses a software
controllable (digital computer) switching apparatus to short out or
remove shorts from electrical connections between the wiper and
first and second terminals of a potentiometer. The switches are
relays, not semiconductor devices such as FET's. Programming as
taught by U.S. Pat. No. 4,849,903 may be used to select relays in
accordance with this invention. FET's will not work with this
potentiometer because they conduct in only one direction and
therefore cannot respond to alternating current measurements or
alternating current control. Still further, FET based devices are
necessarily connected so that the polarity is correct.
[0009] In this invention, a string of resistors having preferably
different values are used with a programmable interface controller
(PIC) chip having a customized code to implement the digitally
controlled potentiometer. The switches are relays which may be
individual relays, double pole double throw relays or double pole
four throw relays. This potentiometer is configurable as a single
potentiometer, or in a pair with master/slave capabilities for a
Wheatstone bridge, stereo audio and other applications where a pair
of potentiometers are required.
[0010] This potentiometer solves several problems associated with
existing potentiometers when used in conjunction with an AC source.
The first problem is resistance precision and drift associated with
mechanical analog potentiometers. This potentiometer allows for
precise setting of resistance especially in the embodiment which
can connect the same resistor between the wiper and either
terminal. A second problem with prior art devices is inability to
handle zero-referenced AC signals as with semiconductor MOSFET
potentiometers. This invention is resistor-based, producing no
effects that interfere with AC wave forms applied to the device. A
third problem is that high-current applications have only been
controlled by mechanical analog potentiometers. Existing MOSFET
potentiometers will fail when currents exceed 200 mA.
[0011] The present invention which provides a variable
potentiometer which uses the different resistors between the wiper
and the first and second terminals which comprises in combination:
a wiper terminal; a first terminal electrically connected to said
wiper terminal by a first group of resistors; a second terminal
electrically connected to said wiper terminal by a second group of
resistors; a plurality of switches for changing resistance between
said wiper terminal and each of said first terminal and second
terminal; wherein resistance is changed between said wiper and said
first terminal by placing a short circuit across one or more of
said first group of resistors or by removing a short circuit from
across one or more of said second group of resistors; wherein
resistance is changed between said wiper and said second terminal
by placing a short circuit across one or more of said second group
of resistors or by removing a short circuit from across one or more
of said second group of resistors; and wherein resistors of said
first group and resistors of said second group have values which
correspond to each other.
[0012] The present invention which provides a variable
potentiometer which uses the different resistors between the wiper
and the first and second terminals wherein the switches comprise
relays which replace resistors with short circuits and replace
short circuits with resistors. The relays may comprise double pole
double throw relays. A computer is used for controlling the
plurality of relay switches. The computer may have a display which
provides for a rotating pointer within a circle which is controlled
by a mouse. The first and second resistor sets are made with
resistors having the same standard value and tolerances.
[0013] The present invention which provides a variable
potentiometer which uses the different resistors between the wiper
and the first and second terminals wherein the first and second
resistors are resistors having a tolerance value of which is
determined so that where the smallest resistor value is R1 and the
largest resistor is Rn, then: Rn.times.(TOLERANCE)<R1
[0014] The present invention provides a variable potentiometer
which uses the same resistors between the wiper and the first and
second terminals which comprises in combination: a wiper terminal;
a first terminal electrically connected to said wiper terminal; a
second terminal electrically connected to said wiper terminal; a
plurality of switches for changing resistance between said wiper
terminal and each of said first and second terminals; wherein when
resistance is reduced between said wiper and one of said first or
second terminals a first resistor is replaced with a first short
circuit; and wherein when resistance is increased between said
wiper and another of said first and second terminals a second short
circuit is replaced with the first resistor.
[0015] The present invention provides a variable potentiometer
which uses the same resistors between the wiper and the first and
second terminals which comprises in combination: a set of
resistors; a wiper terminal; a first terminal electrically
connected to said wiper terminal; a second terminal electrically
connected to said wiper terminal; a plurality of switches for
changing resistance between said wiper terminal and each of said
first and second terminals; wherein when resistance is reduced
between said wiper and one of said first or second terminals a
subset of said resistors is replaced with first short circuits; and
wherein when resistance is increased between said wiper and another
of said first and second terminals second short circuits are
replaced with the subset of resistors.
[0016] The present invention may use switches which comprise relays
which replace the first resistor with a short circuit and replace a
short circuit with the first resistor. The present invention may
use single pole single throw, double pole double throw or four pole
double pole relays.
[0017] The present invention may change resistance between the
wiper and one of said first and second terminals in the case where
the same resistor is used between the wiper and the first or second
terminals wherein: a first subset of double pole double throw
relays electrically remove the subset of resistors from the
electrical connection between one of said first and second
terminals; a second subset of double pole double throw relay insert
a subset of shorting connections in the electrical connection
between the one of said first and second terminals; the first
subset of double pole double throw double throw relays electrically
insert the subset of resistors between the other of said first and
second terminals; and the second subset of double pole double throw
relays remove a subset of short connections between the other of
said first and second terminals.
[0018] The present invention may change resistance in a variable
potentiometer which uses the same resistors between the wiper and
the first and second terminals wherein when resistance is changed
between said wiper and one of said first and second terminals: a
subset of first double pole double throw relays which electrically
remove a resistor subset from the electrical connection between one
of said first and second terminals and replace the resistor subset
with a subset of shorts; a subset of second double pole double
throw relays which electrically insert the resistor subset and
remove a subset of shorts in the electrical connection between the
other one of said first and second terminals.
[0019] In the present invention when resistance is reduced between
the wiper and a terminal and when resistance is increased between
the wiper and another terminal, a subset of the plurality of
resistors are replaced with first short circuits and a
corresponding subset of second short circuits are replaced with the
subset of resistors.
[0020] In the present invention, the resistors in the set of
resistors have a tolerance value of which is determined so that
where a smallest resistor value is R1 and a largest resistor value
is Rn, then: Rn.times.(TOLERANCE)<R1
[0021] In the present invention, the sum of the set of resistor
values is constant and the total resistance between said first and
second terminals always constant.
BRIEF DESCRIPTION OF THE DRAWINGS
[0022] FIG. 1 shows conceptually a potentiometer in accordance with
Applicant's invention where pairs of resistors are used between a
fixed wiper and terminals T1 and T2. Each pair of resistors has the
same value.
[0023] FIG. 2 shows an implementation of the device of FIG. 1 where
switches for pairs of resistors are controlled by
double-pole-double-throw relays.
[0024] FIG. 3 shows a second embodiment of Applicant's invention
where a single resistance is used on either side of the wiper
terminal. In this embodiment the connections to the string are
connected to the pole contacts of relays, and shorts and resistors
which are moved from one side of the wiper terminal to another.
[0025] In FIG. 4, there is shown another embodiment utilizing a
four pole double throw switch relay where the pole contacts are
connected respectively to resistors and shorts. The throw contacts
are connected to points in the wiper.
[0026] In FIG. 5a there is shown a linear monotonic relationship
between a selected resistance value and an actual value. Monotonic
defines the relationship as always increasing in value.
[0027] In FIG. 5b there is shown a non-linear monotonic
relationship between a selected resistance value and an actual
value. Monotonic defines the relationship as always increasing in
value, but since it is non-linear the slope of the line while
always positive can change.
DETAILED DESCRIPTION
[0028] The invention may use a serial port to precisely set
resistance values using a defined control sequence sent to the
potentiometer device over a serial port. Actual switching of
resistor shorts or resistor position from one side of a wiper to
another can be easily implemented by relays controlled by the
computerized sequence. This device when fully implemented may
display the requested resistance on a computer screen or a display
device incorporated into a test instrument. The potentiometer can
be paired with a "slave" potentiometer to create a Wheatstone
bridge circuit that works using either AC or DC signals.
[0029] The potentiometer of this invention which utilizes resistors
and relays will have electrical characteristics exactly like a
standard analog potentiometer in that there is no need to consider
the polarity or absolute sign of the signal applied. In this
invention, the speed at which the resistance can be changed is
limited by the speed of the micro-mechanical relays. The frequency
response is limited by stray capacitance and is generally above 1
Mhz. Ordinary relay response times exceed the response time for
hydraulic systems.
[0030] This invention allows for easy software implemented minimum
increment changes or software implemented major changes without
passing through intermediate values (software-selectable resistance
values), constant input impedance, current limits are set by
resistor ratings, two board designs (master and slave), and
maintenance of last resistance value in non-volatile memory (last
relay state).
[0031] In the case where separate resistors are used between the
wiper and a first terminal and the wiper and a second terminal, the
digital potentiometer is constructed from a pair of resistor sets
and a set of electromechanical relays. There may be two circuits
(master and slave) which share a serial port input which provides
control information (from the computer) regarding how the
circuit(s) should behave electrically. Each circuit "listens" to
the serial port for control information that determines which
circuit is enabled, and which resistance value that circuit should
select. If a circuit is enabled, the PIC chip on the circuit board
determines the requested resistance from the control information.
The software in the PIC chip then looks up the configuration of
relays that should be opened or closed so that the resister series
output resistance is the value requested by the computer which
effectively creates a "wiper" as used in analog potentiometers. The
"wiper" of the potentiometer is "moved" by changing the state of
relays (pairs of single pole single throw) or the state of a double
pole double throw relay. The circuit operates electrically like a
conventional potentiometer in all important characteristics,
including fixed resistance across the potentiometer (within
tolerance limits). The number of resistors in the string, and the
number of strings, is determined by the overall resolution
required, i.e., 1 k, 10 k, 100 k.
[0032] Where the present invention provides a variable
potentiometer which uses the same resistors between the wiper and
the first and second terminals an electromechanical relay removes a
resistor and replaces it with short circuit between the wiper and a
first terminal and removes a short circuit and replaces it with the
resistor between the wiper and a second terminal. Single pole
single throw, double pole double throw or four pole double pole
relays may be used. This configuration avoids tolerance problems
which are unavoidable where different resistors are used between
the wiper and different terminals. Otherwise, this embodiment is
controlled like the case where separate resistors are used between
the wiper and a first terminal and the wiper and a second
terminal.
[0033] FIG. 1 shows conceptually the switches associated with
resistors R1, R2, Rn, Rn', R2' and R1'. Where R1 is in the circuit
to the side of terminal T1, R1' to the side of the terminal T2' is
short circuited, thereby removing R1' from the T2 side of the wiper
W. In a similar manner, whenever any designated resistor is short
circuited on one side of the wiper, its corresponding resistor is
in circuit or is no longer short circuited on the other side of the
wiper. As shown in FIG. 1, relays Rly1, Rly2 . . . Rly1', Rly2'. .
. are single pole single throw relays which may be controlled by a
computer. The only necessary condition is that when a relay such as
Rly1 is closed, that Rly1' must be open. In this manner, if
resistors R1=R1', R2=R2', and Rn=Rn', then the resistance from
terminal T1 to terminal T2 will always remain constant.
[0034] FIG. 2 is a double-pole-double-throw relay embodiment of the
device shown in FIG. 1. As shown in FIG. 2, relay Rlyn short
circuits resistor Rn and opens a short across resistor Rn', thereby
inserting resistor Rn' between W and T2. On the other hand, relay
RLY1 short circuits resistor R1' and opens a short across resistor
R1, thereby inserting R1 into the connection between W and T1.
[0035] However, as a practical matter, resistors necessarily have
tolerance limitations. For instance, all resistors should have
close tolerances, otherwise if resistors having large values also
have large tolerances, the shorting out of resistor Rn on the T1
terminal side of the potentiometer of FIG. 1 and removing the short
from Rn' in the T2 terminal side of the potentiometer of FIG. 1
will result in a variance which is greater than the total amount of
a low value resistor such as R1 and R1'.
[0036] Resistors R1, R2, . . . Rn may vary in accordance with the
following formula: Rn=R1 (2.sup.n-1)
[0037] If resistors in the string on the T1 side of the wiper and
the T2 side of the wiper of FIG. 1 are exactly equal in value (no
tolerance variation), then substitution of R1' for R1 between T1
and T2 will cause no variance in the overall resistance of the
wiper W. Considering the resistance between the wiper W and T2,
this resistance may be changed by any combination of the number of
resistors R1', R2' . . . Rn'.
[0038] If the resistor values and the resistors value selected are
perfect (no tolerance variation), an increase in selected value (R)
will produce an actual increase in resistance which is directly
proportional to the selected value as shown in FIG. 5a. This
relationship shown in FIG. 5a is generally referred to as a linear
relationship. In such a linear relationship, as a value is
selected, the actual value always increases and at the same rate.
There is also the case of a non linear relationship, but monotonic
relationship which is shown in FIG. 5B. In this case, selected
values of R do not result in straight line linear increases in the
actual value of R, but the actual value of R never decreases as a
higher selected value of R is chosen.
[0039] In the embodiment of FIG. 1 and FIG. 2 it is necessary to
maintain a monotonic relationship so that it is known that when
resistance is changed, in an arm, such as from W to T2, that actual
resistance increases. Conversely, if resistance is decreased from W
to T1, it must also follow a monotonic curve a shown in FIGS. 5a
and 5b. If the tolerances in the resistors R1'-Rn' are large, then
the selection of a higher value of R may actually result in a
reduction of the actual R. This condition will result in erroneous
measurement and monotonicity is lost due to high tolerance
variations in the individual resistors.
[0040] In the embodiments shown in FIGS. 1 and 2, if R1 is the
smallest resistor and the largest resistor is equal to Rn, then the
relationship is monotonic if Rn.times.(TOLERANCE)<R1
[0041] An illustration of the problem of maintaining monotonicity
(ever increasing value of R) is illustrated by considering a string
of resistors which follow the relationship Rn=R1 (2.sup.n-1)
[0042] If the total resistance in the string (from W to T1 on W to
T2 is 1023 ohms, then the highest resistance value will be a
resistor of 512 ohms. The 512 ohm resistor and the sum of all the
lower value resistors will total 1023 ohms. In selecting the
resistance, if the previous sum of 511 ohms is correct, and a step
up to 512 ohms is taken, the 511 ohm string of resistors will all
be shorted and the 512 ohm resistor will be inserted. If the 512
ohm resistor has a tolerance of 1%, it is possible that the
selection will result in a resistor of the value of 512-5.12 or
506.88 ohms. This would be a reduction in the resistance value of
the string and violate the monotonicity requirement as illustrated
in FIGS. 5A and 5B because the actual value of R must always
increase. However, as illustrated here, the value can actually
decrease if tolerance is 1%. 5.12 ohms is greater than the initial
1 ohm resistor which is used in making up values of the actual
resistance. The value of 513 ohms (512+1) utilizing the 1%
tolerance 512 ohm resistor would result in 507.88 ohms which is
still less than the value intended. Under these conditions, the
operator of an instrument will be under the impression that the
resistance is increasing when it is in fact decreasing. This
problem can be solved by tightening the tolerance on the 512 ohm
resistor to 0.1% which results in 512 ohms-0.512 ohms. Monotonicity
is achieved although it is nonlinear.
[0043] The embodiments shown in FIGS. 3 and 4 completely solve the
problem of tolerance values and monotonicity as discussed above.
The solution requires the use of exactly the same resistors between
W and T1 and W and T2. As shown in FIG. 3, each resistor R1, R2 . .
. Rn is electrically removed from the string from T1 to W and
inserted into the string from W to T2 upon actuation of the
associated relay. The resistor R1 of FIG. 3 is shown in the
condition where R1 is inserted into the string between W and T2. By
utilizing the exact same resistors, even if tolerances are loose,
necessarily the resistance will always be reduced on one side and
always increased on the other side of the wiper because the
resistor remains the same. Therefore the embodiments shown in FIGS.
3 and 4 solve the tolerance problem and provide at least a
nonlinear but monotonic relationship in all cases. If the
resistance tolerances are very small in the devices of FIGS. 3 and
4, then the relationship between the selected resistor and the
actual resistor will approach that of a linear relationship as
shown in FIG. 5a.
[0044] The relays of this invention may be wired to the short
circuits and to the resistors in any manner so that when the relays
are closed (normally closed, no power), there is any combination of
resistances between W and T1 and W and T2. As shown in FIG. 3, the
resistance between W and T1 is Rn and the resistance between W and
T2 is R1. By wiring the relays so that resistances are not 0
between either W and T1 and W and T2, the operator is assured that
there will not be a short circuit between W and either one of the
terminals upon initial startup. This has advantages in certain
types of measurements and instruments. This is a hard wired
solution which is implemented in accordance with this
invention.
[0045] Resistance values for resistors following the relationship
R=R1 (2.sup.n-1) can be selected from standard resistance values
which are commonly available. Table 1 shows a standard resistance
value table which shows values which are readily obtainable.
TABLE-US-00001 TABLE 1 .39 3.90 39 390 3900 .43 4.30 43 430 4300
.47 4.70 47 470 4700 .51 5.1 51 510 5100 .56 5.6 56 560 5600 .62
6.2 62 620 6200 .68 6.8 68 680 6800 .75 7.5 75 750 7500 .82 8.2 82
820 8200 .91 9.1 91 910 9100 1.00 10.0 100 1000 10000 1.10 11.0 110
1100 11000 1.20 12.0 120 1200 12000 1.30 13.0 130 1300 13000 1.50
15.0 150 1500 15000 1.60 16.0 160 1600 16000 1.80 18.0 180 1800
18000 2.00 20.0 200 2000 20000 2.20 22.0 220 2200 22000 2.40 24.0
240 2400 24000 2.70 27.0 270 2700 27000 3.00 30.0 300 3000 30000
3.30 33.0 330 3300 33000 3.60 36.0 360 3600 36000
[0046] An example of selection of resistors to yield a relationship
R=R1 (2.sup.n-1) is shown in Table 2. TABLE-US-00002 TABLE 2 2n
Standard Resistor Values 1 1 2 2 4 2 + 2 8 3 + 3 + 2 16 16 32 16 +
16 64 62 + 2 128 100 + 27 + 1 256 240 + 16 512 510 + 2
[0047] The resistance values for the embodiment of FIGS. 1 and 2,
or for the embodiments of FIGS. 3 and 4 may also be selected in
accordance with any criteria, such as all equal, or in arbitrary
units such as combinations of 1, 5, 10, 50, 100, 500, and/or 49.5
ohms.
* * * * *